In recent years, power supply systems utilizing distributed power supplies such as gas engine generators and fuel cells are drawing increasing attention. For example, there is one proposed system that uses a power generator called a distributed power supply mentioned above to supply electric power from the power generator to a plurality of electrical loads in a particular area. Such a system, which is called a micro grid, is often configured such that a power system therein is connected not only to the aforementioned power generator but also to an energy storage that is equipped with, for example, a secondary battery. The energy storage is provided for compensating for fluctuations in power generation by the power generator as well as load variations. There is also a case where the power system is an AC power system, but the power generator is a DC power generator. In this case, a power conversion device for AC-DC power conversion is necessary.
Such a system utilizing a distributed power supply often assumes interconnected operation in which the system is operated while being interconnected with a commercial electrical grid (see Patent Literatures 1 to 3, for example).
However, there are cases assuming both the interconnected operation and self-sustained operation in which the system is operated independently of the commercial electrical grid. In such a case, it is important to realize stable power supply in both of these operations. Also, the power conversion device is required to have functions that make such stable power supply possible.
In view of the above, the applicant of the present application has recently developed a technique of controlling a power conversion device that has characteristics equivalent to those of a grid-interconnected power generator (i.e., a power conversion device with a generator model). The applicant has proposed a power conversion device that realizes: stable power supply not only during interconnected operation but also during self-sustained operation; and a shift from interconnected operation to self-sustained operation without requiring a changeover of control (see Patent Literature 4). In addition, as disclosed in Patent Literature 5, the applicant has also proposed a technique of stabilizing self-sustained operation by such a power conversion device.